1. Field of the Invention
The invention relates to the cancer screening method, cancer screening biomarker, and use thereof. In particular, to the cancer screening method using methylated DNA as the biomarker.
2. Description of the Prior Art
Cervical cancer has been one of the main causes of death in females worldwide and in Taiwan. Based on the statistical survey by the World Health Organization (WHO) in 2002, cervical cancer was the second major disease responsible for the death of women worldwide, second to breast cancer. Regular cervical cancer screening is the best way to prevent cervical cancer. Conventional cervical cancer screening includes two approaches: the most commonly used Pap smear, and human papilloma virus testing (HPV testing). Pap smear testing for early detection of cervical cancer consists of sampling secreta from cervix uteri then examining the sample under a microscope to determine whether there is cancerous pathological change in the exfoliated epithelial cell. On the other hand, HPV testing consists of examining whether human papilloma virus (HPV) is present in the specimen by using polymerase chain reaction (PCR) or Hybrid Capture.
There are, however, many undesired properties of Pap smear testing. For one, it requires sampling by a physician, and analysis by a medical examiner/pathologist, which has a high manpower cost that poses difficulty in promoting the test in many developing countries. Also, Pap smear has a high false negative rate which delays diagnosis and proper treatment prior to cancerous pathological change. As for HPV testing, although it is highly sensitive, it tends to create a high false positive rate, which not only makes patients worry in vain, but also wastes medical resources in follow up examinations resulting from those false positive patients. Accordingly, one of the important topics in promoting cervical cancer examination relies on increasing the accuracy and convenience of cervical cancer examination methods.
Genomic deletions have long been considered to be an important factor in tumorigenesis. For a long time, we have been accustomed to the idea that the coding potential of the genome lies within the arrangement of the four A, T, G, and C bases. The two-hit theory proposed as early as in 1970's indicates concomitant mutations or deletions of some homologous tumor suppressor genes may cause or predispose to cancer development. However, additional information that affects phenotype can be stored in the modified base 5-methylcytosine. 5-Methylcytosine is found in mammals in the context of the palindromic sequence 5′-CpG-3′. Most CpG dinucleotide pairs are methylated in mammalian cells except some areas called “CpG island.” CpG islands are GC- and CpG-rich areas of approximately 1 kb, usually located in the vicinity of genes and often found near the promoter of widely expressed genes. Cytosine methylation occurs after DNA synthesis, by enzymatic transfer of a methyl group from the methyl donor S-adenosylmethionine to the carbon-5 position of cytosine. The enzymatic reaction is performed by DNA methyltransferases (DNMTs). DNMT1 is the main methyltransferase in mammals, and is responsible for the post-replicative restoration of hemi-methylated sites to full methylation, referred to as maintenance methylation, whereas DNMT3A and DNMT3B are thought to be involved primarily in methylating new sites, a process called isolated methylation.
Loss of methylation at CpG dinucleotides, i.e., general hypomethylation, was the first epigenetic abnormalities identified in cancer cells. However, during the past few years, it has become increasing apparent that site-specific hypermethylation, e.g., some tumor suppressor genes, is associated with loss of function which may provide selective advantages during carcinogenesis. Dense methylation of CpG islands at promoter regions can trigger chromatin remodeling through histone modifications with subsequent gene silencing. Therefore, in addition to chromosomal deletions or genetic mutations, epigenetic silencing of tumor suppressor genes by promoter hypermethylation is commonly seen in human cancer.
Epidemiologic studies have recently shown the correlation of serum folate level, a major source of methyl group, with the infection and clearance of HPV. Genetic polymorphisms of enzymes in the metabolism of methyl cycle were also reported to be associated with the development of cervical intraepithelial lesions. As the concept of epigenetics evolves, studies exploring the association between DNA methylation and cervical cancer are also booming. Studies of DNA methylation in cervical cancer are accumulating, which showed the potential of using methylation as markers in cervical screening. With the nature of the interface between genetics and environment, the prevalence of methylation in tumor suppressor genes varies in different genes and different populations. The concept of methylator phenotypes with different disease behaviors was proposed with controversy. The methylator phenotype of cervical cancer and its interaction with HPV genotypes still remains unknown. What genes are specifically methylated in cervical cancer and how many genes are required to achieve clinical application will remain a blossoming issue in the coming future.
The inventor of this application had filed relative patent applications in Taiwan (TW Pat. Pub. No. 200831900) and America (US Pat. Pub. No. 20080311570) (Hereinafter referred as prior applications). Comparing with prior applications, the inventor had found some novel cancer screening biomarker and cancer screening method using the same. Further, the research and development result of partial technology in this application had been disclosed publicly in AACR (American Association for Cancer Research) Annual Meeting 2009 held on Apr. 22, 2009.